Think this may be a possible bug. When you have 2 persons riding a mega and you cross a sim border, the riders swap positions so the passenger rider becomes the driver, and vice versa. No idea what causes it but I managed to replicate it a few times to make sure it wasnt just a one off lag occurrence.
PS: I dont find this to be an urgent bug if it is one at all, which is why I decided to post it here instead of a support ticket